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The present work is focused on the studies of the phase-structural transformations in the La 3-x Mg x Ni 9 (x ​= ​1.0, 1.1 and 1.2) alloys as active materials of negative electrodes in the Nickel-Metal Hydride (Ni/MH) batteries. The phase equilibria and phase-structural transformations in the alloys were probed by in situ neutron powder diffraction (NPD) at the temperatures ranging from 300 ​K to 1273 ​K using the measurements of the equilibrated alloys at 8 setpoint temperatures of 300, 973, 1073, 1123, 1173, 1223, 1248 and 1273 ​K. Prepared by induction melting initial alloys were found to be multi-phase structured, containing up to 6 individual intermetallic compounds with different stoichiometric compositions. With the increase of the temperature and holding time, various transformations took place in the studied alloys. These included the formations and transformations of super-stacking intermetallics with variable ratios (La ​+ ​Mg)/Ni, 1:3, 2:7 and 5:19. With increasing temperatures, several systematic changes took place. (a) Abundances of (La,Mg) 2 Ni 4 AB 2 and (La,Mg)Ni 3 AB 3 type intermetallics gradually decreased before they melted/decomposed above 1073 ​K; (b) The (La,Mg) 2 Ni 7 A 2 B 7 type intermetallics began to decrease in abundances above 1123 ​K; (c) The transformation in the (La,Mg) 5 Ni 19 intermetallics from 3R to 2H proceeded above 1223 ​K. The increase of Mg content had no obvious influence on (La,Mg) 2 Ni 4 and (La,Mg) 2 Ni 7 phases, and corresponding reactions R1 and R3 took place at the same temperatures as in the La–Ni system. However, with increasing Mg content the melting point of (La,Mg) 5 Ni 19 phase increased while the melting point of the (La,Mg)Ni 3 phase it decreased, leading to the variation of the reaction temperatures of the corresponding processes. The present study will assist in optimizing phase-structural composition of the alloys in the La–Mg–Ni system which contain Mg-modified layered structures by tailoring the high temperature annealing conditions. [Display omitted] • Transformations in the La 3-x Mg x Ni 9 (x ​= ​1.0, 1.1 and 1.2) alloys were studied by in situ NPD at 300–1273 ​K. • Initial alloy is multi-phase structured containing up to six different intermetallics with five stoichiometric compositions. • Various transformations take place on heating, leading to the disappearance of LaNi5 and LaMgNi 4. • The increase of Mg content has different influences on the intermetallics and affects on the five peritectic reactions. • The results guide optimization of phase-structural composition of La–Mg–Ni alloys by tailoring high temperature annealing conditions. [ABSTRACT FROM AUTHOR]

Evaluation of the performances of hydrogen storage systems accommodating solid H storage materials should include characteristics on their reversible hydrogen storage capacity, operating pressures and temperatures, packing densities, and heat effects of hydrogen uptake and release. We have conducted a performance evaluation of the systems accumulating 5 kg of hydrogen in a containment of cylindrical geometry filled with a solid H storage material including such hydrides and reactive hydride composites as AlH 3 , MgH 2 , “low-temperature” (inter)metallic hydrides, NaAlH 4 , Na 3 AlH 6 , LiBH 4 + MgH 2 , and MOFs. The analysis yielded gravimetric and volumetric H storage capacities, and energy efficiencies of hydrogen stores. We conclude that the weight efficiency of hydrogen stores, apart from the gravimetric H storage capacity of the material, is greatly affected by its packing density, and by the pressure–temperature conditions which determine type and dimensions of the containment. The materials with low heat effects of H exchange, operating close to the ambient conditions, should be targeted in the course of the development of new hydrogen stores as offering the best energy efficiency of their operation. [ABSTRACT FROM AUTHOR]

Abstract: “Hybrid” hydrogen storage, where hydrogen is stored in both the solid material and as a high pressure gas in the void volume of the tank can improve overall system efficiency by up to 50% compared to either compressed hydrogen or solid materials alone. Thermodynamically, high equilibrium hydrogen pressures in metal–hydrogen systems correspond to low enthalpies of hydrogen absorption–desorption. This decreases the calorimetric effects of the hydride formation–decomposition processes which can assist in achieving high rates of heat exchange during hydrogen loading—removing the bottleneck in achieving low charging times and improving overall hydrogen storage efficiency of large hydrogen stores. Two systems with hydrogenation enthalpies close to −20kJ/mol H2 were studied to investigate the hydrogenation mechanism and kinetics: CeNi5–D2 and ZrFe2−x Al x (x =0.02; 0.04; 0.20)–D2. The structure of the intermetallics and their hydrides were studied by in situ neutron powder diffraction at pressures up to 1000bar and complementary X-ray diffraction. The deuteration of the hexagonal CeNi5 intermetallic resulted in CeNi5D6.3 with a volume expansion of 30.1%. Deuterium absorption filled three different types of interstices, Ce2Ni2 and Ni4 tetrahedra, and Ce2Ni3 half-octahedra and was accompanied by a valence change for Ce. Significant hysteresis was observed between deuterium absorption and desorption which profoundly decreased on a second absorption cycle. For the Al-modified Laves-type C15 ZrFe2−x Al x intermetallics, deuteration showed very fast kinetics of H/D exchange and resulted in a volume increase of the FCC unit cells of 23.5% for ZrFe1.98Al0.02D2.9(1). Deuterium content, hysteresis of H/D uptake and release, unit cell expansion and stability of the hydrides systematically change with the amount of Al content. In the deuteride D atoms exclusively occupy the Zr2(Fe,Al)2 tetrahedra. Observed interatomic distances are Zr–D=1.98–2.11; (Fe, Al)–D=1.70–1.75Å. Hydrogenation slightly increases the magnetic moment of the Fe atoms in ZrFe1.98Al0.02 and ZrFe1.96Al0.04 from 1.9μB at room temperature for the alloy to 2.2μB for its deuteride. [Copyright &y& Elsevier]

This work was focused on studies of temperature-induced phase-structural transformations in the as-cast La 2 MgNi 9 metal hydride battery electrode alloy. The interactions in the alloy were studied by in situ neutron powder diffraction at temperatures ranging from 300 K to 1273 K. Initial alloy is multi-phase structured, containing six different intermetallics with five stoichiometric compositions. These include the targeted rhombohedral La 2 MgNi 9 as the main phase constituent, together with three electrochemically active intermetallics, La 3 MgNi 14 (3R), La 4 MgNi 19 (2H) and La 4 MgNi 19 (3R). Furthermore, the alloy contained two “ballast” intermetallics, LaNi 5 and LaMgNi 4 . Various transformations take place on heating, leading to the significant changes in the contents of the constituent intermetallics, first of all to the disappearance of LaNi 5 and LaMgNi 4 . Thermal volume expansions of the studied intermetallics were well fitted by the linear dependences and resulted in similar values of 4.3–5.5 vol.% at 1223 K as compared to 300 K. [ABSTRACT FROM AUTHOR]